Liquid loading of the wellbore is often a serious problem in aging production wells. Operators commonly use beam lift pumps or remedial techniques, such as venting or “blowing down” the well to atmospheric pressure to remove liquid buildup and restore well productivity. In the case of blowing down a well, the process must be repeated over time as fluids reaccumulate, resulting in additional methane emissions.
Plunger lift systems are a cost-effective alternative to both beam lifts and well blowdowns and can make use of well energy to lift liquid from the well efficiently, i.e., to lift liquid with little or no slug fallback so that gas can flow without the obstruction of liquid loading for a period of time before the plunger is allowed to fall again. A plunger lift system is a form of intermittent gas lift that uses gas pressure buildup in the casing-tubing annulus and surrounding reservoir to push a plunger, and a column of fluid ahead of the plunger, up the well tubing to the surface. The plunger serves as a piston between the liquid and the gas, which minimizes liquid fallback, and acts as a scale and paraffin scraper.
The operation of a plunger lift system relies on the natural buildup of pressure in a gas well during the time that the well is shut-in, i.e., not producing. The well shut-in pressure must be sufficiently higher than the sales-line pressure to lift the plunger and liquid load to the surface. A surface valve is controlled by a microprocessor for controlling the on and off time of the plunger lift system during periods when gas is vented to the sales line or when the well is shut-in. The controller is normally powered by a solar recharged battery and can be a simple timer-cycle or have solid state memory and programmable functions based on process sensors.
During the off times, casing and tubing pressure build as the plunger falls through gas and liquid and then rests on a bumper spring at the bottom of the well. While the well is open, the plunger and liquid rises and the liquid is produced. The plunger is held in the top of the well during an after-flow period by gas flow. As the gas flow diminishes below a critical value, liquid begins to accumulate in the bottom of the tubing. Liquid accumulated in the bottom of the tubing is evidenced by surface measurements that show casing pressure being higher than tubing pressure during the shut in period.
Operation of a typical plunger lift system involves the following steps: The plunger rests on a bottom hole bumper spring located at the base of the well. As gas is produced to a sales line, liquids accumulate in the well-bore, creating a gradual increase in backpressure that slows gas production. To reverse the decline in gas production, the well is shut-in at the surface by an automatic controller. This causes well pressure to increase as a large volume of high pressure gas accumulates in the annulus between the casing and tubing. Once a sufficient volume of gas and a sufficient pressure is obtained, the plunger and liquid load are pushed to the surface. As the plunger is lifted to the surface, gas and accumulated liquids above the plunger flow through the upper and lower outlets. The plunger arrives and is captured in the lubricator, situated across from the upper lubricator outlet. The gas that has lifted the plunger flows through the lower outlet to the sales line. Once gas flow is stabilized, the automatic controller releases the plunger, dropping it back down the tubing. The cycle repeats. The above is known as a plunger cycle.
Overall control of a plunger cycle can be implemented in different ways. One simple way involves opening a control valve when high casing pressure is experienced and flowing gas and liquid until a low casing pressure is achieved. Alternatively, the control valve may be opened when a high tubing pressure is experienced or the control valve may be closed when a low tubing pressure is experienced. These simple methods may require trial and error to get to continuous repeating cycles, i.e., to prevent the well from becoming liquid loaded.
Another example of an overall control algorithm involves monitoring rise velocity of the plunger and liquid. Experience has shown that arrival between 500-1000 fpm is a good operating range. Using this method, if the plunger and liquid come up faster than 1000 fpm, then the controller may be instructed to shut in for a shorter time during a following cycle, which would result in less casing pressure to lift the plunger and liquid. However, the controller must still facilitate a shut in that is long enough for plunger to fall to bottom of the well. Additionally, the well could flow longer during a following cycle, accumulating more liquid to make the plunger and liquid rise more slowly, i.e., within the range of 500-1000 fpm, as longer flow time below critical accumulates more liquid in the tubing. In this example, the controller looks at the current cycle and makes recommendations for timing of control valve opening and closing for the next cycle.
Using the same method, if the plunger were to rise too slowly, then the shut in time may be increased on the next cycle to give more casing pressure to lift the plunger more quickly. Alternatively, the flow time may be decreased to lift a smaller liquid slug. However, the flow time for the next cycle must be long enough to accumulate some liquid because if no liquid is accumulated, then the plunger will rise too fast and may cause damage.
The above are examples of overall cycle control. However, the control depends on the current cycle performance for making operational recommendations for the next cycle. A potential drawback is that too much liquid is accumulated in a current cycle, resulting in the plunger not rising in the next cycle, i.e., loading the well. Alternatively, the liquid slug may be too small and the plunger will rise too fast in the current cycle and do damage before adjustments are made.
New information technology systems have streamlined plunger lift monitoring and control. For example, technologies such as online data management and satellite communications allow operators to control plunger lift systems remotely, without regular field visits. Operators typically visit only the wells that need attention, which increases efficiency and reduces cost.